1
\$\begingroup\$

Disclaimer: I am a total beginner in electronics, no professional degree whatsoever.

I have designed the below 1 cell Li-ion battery protection/charging/buck-boost circuit and I've also designed the PCB for it too.

I am really new in electronics and from what I can see after assembling the PCB:

  • The charging circuit seems to be working. I can connect a Li-po battery and it will charge to 4.2v following the charging curve. The Red and Green LEDs are lighting up correctly.
  • The Buck/Boost circuit doesn't work, and I have no idea why. The symptom is that when I connect the lab bench power supply and set it to 4.2V the output is 3.4V, but the output voltage seems to be dropping with the input voltage instead of staying on ~3.3V.

Related Datasheets for the design:

Few things I can think of:

  • The Buck/Boost IC is too far away from the inductor?
  • I didn't properly solder the QFN10 3x3 converter?

A screenshot of the schematics is attached here, for PCB design please check the EasyEDA link.

Thanks everyone in advance!

Power Supply

\$\endgroup\$
  • \$\begingroup\$ Please be more detailed about the symptoms. \$\endgroup\$ – Andy aka Aug 9 at 7:03
  • \$\begingroup\$ ”IC is too far away from the inductor?” Please post layout! \$\endgroup\$ – winny Aug 9 at 8:13
  • \$\begingroup\$ @Andyaka When the input voltage of the IC is 4.2v, it produces 3.3v as expected. When then voltage start to drop, 4.1v outputs 3.2v, 4.0v outputs 3.1v... and when input drops to 2.7v, output drops to around 1v... I think as José Manuel Ramos mentioned I should test it in isolation :) \$\endgroup\$ – xinsnake Aug 9 at 21:39
  • \$\begingroup\$ @winny Feel free to check the PCB layout in the EasyEDA link I posted. Thanks! \$\endgroup\$ – xinsnake Aug 9 at 21:40
  • \$\begingroup\$ Feel free to post it as an image here, or few will bother. \$\endgroup\$ – winny Aug 9 at 22:21
1
\$\begingroup\$

Welcome to Electrical Engineering stack exchange.

Prior to design and mount the entire pcb, you must test every "module" in an isolated environment firstly. Don't believe your circuit will work as expected in the first try. It probably will never do. Hardware is not software in the real world: your theorically working circuit will crash in the real world in obscure and bizarre ways you will never expect. Cables are antennas, inductances, resistors and capacitors, every non-linear component will have high order components will make you suffer, digital and analog are enemies in the PCB...

If you say your DC/DC doesn't work, it means the DC/DC probably has a design error. So, the procedure to check correctly that module is:

  • Review your requirements. Your DC/DC has to be up to 3.3 volts? 5 Volts?

  • Isolate the DC/DC firstly

  • Analize the input prior to connect to the DC/DC. Is your input correctly defined against your requirements? Good if so, check it again if not.

  • Connect the DC/DC module without any other modules. Which voltage is in the input? Is not the same as before, when you didn't connect the module? If are equals, good. If not, your power supply will not be acceptable to operate with the currents you are working-probably you will have a shortcircuit somewhere

  • Check the output of the DC/DC module now. Which voltage are you measuring? Its the same as the voltage you expect? If not, probably your module will not have the correct feedback values.

At this point, you will need to check if all electrolitic capacitors are in good state and their '+' simbols are placed in the right place.

Checking the datasheet against your circuit, I observe your feedback resistors are not good.

Sample circuits

Your R1 resistor in the sample (R9 in your schematic) is not the same value: 820kohm.

The datasheet does not contain high frequency filters-capacitors in the power lines lower than 10uF. Put a 100nF and a 100pF in parallel, too. It will filter high frequencies and, therefore, add more stability in the inductor when it creates the output voltage. The C3 and C4 in the sample circuits with two more smaller capacitors there, and another pair in the C1/C2 caps, too.

If, with all of these advices, your DC/DC does not work in the PCB with all the modules ready to be soldered, redesign in a separate module from scratch and test the PCB to ensure you will not have any short-circuits without soldering any component. Then, if your PCB does not have any short circuit, solder the components and measure the DC/DC nicely module you will have. If the DC/DC goes well at this point, and it works as expected, probably you will have a bad shielding (bad design of ground planes) in your circuit and you will need to isolate the power planes.

\$\endgroup\$
  • \$\begingroup\$ Thanks very much for your detailed comment! I think you are correct as a software engineer I tend to think things will work as expected, big mistake! \$\endgroup\$ – xinsnake Aug 9 at 21:41
  • \$\begingroup\$ I intended to have this running on 3.3v output. Thanks very much for your inspection on the capacitors. Regarding the resistor, I couldn't find an 820k so I used a 787k/137k for that. According to the datasheet using the formula, I should get a similar result (R9/R10=5.74 vs 820k/143k=5.73). Please correct me if I am wrong here. \$\endgroup\$ – xinsnake Aug 9 at 21:47
  • \$\begingroup\$ I have a feeling the problem is I don't have a common GND as reference. I should have connected U1 PIN6 to GND instead of Battery -? I can see that when I am putting 4V in the battery +/-, I get only 0.9V when measuring the voltage across C5. \$\endgroup\$ – xinsnake Aug 9 at 22:02
0
\$\begingroup\$

I have definitely had issues with the soldering of small QFN's. I would recommend producing at least 3 of DC/DC circuit, to see if your soldering improves. Just starting over with a new board has helped me at least a couple of times.

Do you use solder paste/stencil for this? Be careful with putting too much paste on the thermal pad.

In debugging this you need to know how stable the switching is. It would be nice the see the inductor current waveform if you have equipment for that. And remember to put some known load on the converter, try both high loads and low loads to see if something improves.

\$\endgroup\$
  • \$\begingroup\$ Thanks for your comment. I have only an oscilloscope and maybe I can use that in some way :) \$\endgroup\$ – xinsnake Aug 9 at 21:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.